Crude oil and related petroleum products are routinely loaded into storage compartments or tanks aboard a tanker ship for transportation to market. In many conventional loading operations, substantial amounts of hydrocarbon vapors or gas may be generated in the tanks during filling due to the vapor pressure of the crude oil and/or the differences in temperatures between the oil and the air and/or inert gas which normally fills the otherwise empty compartment at the beginning of the loading operations. As a compartment or tank is filled, these hydrocarbon vapors are forced out the top of the compartment along with the air or inert gas by the incoming crude. In addition to the possible significant economic loss due to the lost vapors, themselves, these discharged vapors also present certain ecological and safety problems. Although, of course, these vapors may be trapped and recovered or burned, the costs involved are substantial and in most cases is usually economically-infeasible to do so in most environments.
In addition to hard-pipe capture systems, several other approaches have been proposed for controlling these vapors during loading operations to alleviate the problems associated therewith. For example, it has been proposed to install "floating roofs" in the compartments wherein the roof rests on and rises with the surface of the crude as it is being loaded thereby effectively eliminating the space above the crude into which the vapors can evolve and accumulate. Another approach is described in U.S. Pat. No. 3,146,060 wherein a layer of microspheres floats on the surface of the crude to adsorb the vapors which are formed during loading. Still another approach involves "gelling" the upper surface layer of the crude which will then function in the same manner as a floating roof within the compartment; see U.S. Pat. No. 3,639,258. Recently, an approach has been proposed wherein the air or inert gas which is present in a compartment at the commencement of the filling operation is heated to a temperature which matches or exceeds that of the incoming crude to thereby reduce the vaporization of the crude; see co-pending U.S. patent application No. 07/497,278, filed Mar. 22, 1990, and commonly assigned to the present assignee.
One approach which appears to be promising from a commercial viewpoint involves spraying an aqueous foam into the top of the compartment and allowing it to form a layer on the bottom of the compartment before loading the crude oil in under the foam layer. The foam floats on and rises with the crude as it fills the compartment and forms a highly effective barrier between the crude and the space above which blocks the hydrocarbon vapors from mixing with the inert gas or air above the foam. Since substantially no vapors are formed, the only hydrocarbon vapor that will be vented from the compartment during filling will be any residual gas which remained in the compartment from a previous cargo. For a more complete description of this method, see U.S. Pat. No. 3,850,206.
In carrying out a commercial operation using foam as described above, a foam generator is mounted on a special Butterworth cover which, in turn, closes a hatch at the top of the compartment. The generator is suspended into the compartment at the top thereof and mixes a surfactant solution with the inert gas present in the compartment to form the foam. The foam exits the generator and falls onto the bottom of the compartment to form a layer thereon before the crude is loaded through a loading manifold. The use of the in situ generator adds substantially to the capital costs of the system and its placement in the plurality of tanks is time consuming which substantially increases the time for loading a tanker.